The present invention pertains to the art of medical diagnostic imaging. It finds particular application in conjunction with xenon absorption enhanced x-ray tomographic imaging equipment and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in conjunction with other gas absorption enhanced CT imaging techniques, other imaging modalities, and the like.
Arterial blood gas concentration is in equilibrium with lung gases that are in intimate contact with the alveoles. These lung gases, denoted as end-tidal gases, are found at the end of the tide of the exhaled breath. By measuring the concentration of a gas in question in these last bits of the exhaled air, the concentration of the gas in the blood can be determined.
The migration of xenon or other gases through the human anatomy has been measured with a series of CT scans. A first or reference CT scan of the series is taken, before xenon is introduced into the breathing gases. Second, third, and subsequent scans are taken at selected later times. The difference between the reference scan taken in the absence of xenon and the later scans is indicative of the spatial position and quantity of absorbed xenon within the patient tissue. From these difference images and the concentration of xenon in the blood as measured from the end-tidal gases, a flow value and a lambda or partition coefficient value are calculated for each pixel or voxel of the CT scan image. The flow is indicative of blood flow in imaged brain tissue and the lambda value is indicative of a partition coefficient or xenon solubility of the brain tissue. Frequently, a confidence image is also derived, which is indicative of the reliability of the flow and lambda value for each image cell based on a statistical analysis of the data from which the flow and lambda values are calculated. Techniques for determining these values are described in detail in parent U.S. Pat. No. 4,793,357, the disclosure of which is incorporated herein by reference, as well as Gur, et al U.S. Pat. No. 4,535,780, issued Aug. 20, 1985.
The data is displayed in graphic images on a video monitor. Frequently, the monitor display is divided into quadrants to display a CT image, the flow image, the lambda image, and the confidence image. Commonly, the operator defines a region of interest with a cursor or joystick on the displayed images. The system software commonly retrieves and averages each flow value within the region of interest from the flow image or each lambda value within the region of interest from the lambda image.
Anatomical homogeneity of gray and white matter over even small tissue volumes or regions of interest is the exception, not the rule. Flow, in particular, is inhomogeneous even within clearly separated gray and white matter dominant regions of the brain. For example, in the cerebral cortex, the gray and white matter regions are too intricately connected to be separated easily by the clinician.
One of the problems with this display format is that the diagnostic value of the average flow values varies with the operator's ability to accurately define an appropriate region of interest. That is, gray matter and white matter have very different flow values, typically differing by about a factor of 3. Accordingly, even a small amount of gray matter in a perceived white matter region of interest or a small amount of white matter in a perceived gray matter region of interest can radically change the average flow values. Similarly, gray matter and white matter also have different lambda values. Gray matter lambdas are typically 0.85 and white matter lambdas are typically 1.3. Accordingly, small amounts of gray matter mixed with white matter can result in an intermediate value of lambda (0.85&lt;.lambda.&lt;1.3), not correct for either gray or white matter. Because the prior art systems do not quantify the percentage of white and gray matter within the region of interest, these inaccuracies in the flow and lambda values may lead to incorrect diagnoses and less than optimal medical treatment.
In accordance with the present invention, a new and improved diagnostic information display technique is provided.